Pulmonary clear cell carcinoid tumor: Another entity in the differential diagnosis of pulmonary clear cell neoplasia

A clear cell variant of primary pulmonary carcinoid tumor is described. The tumor arose in a 53-year-old woman who was incidentally found to have a solitary pulmonary nodule in the left upper lobe during routine chest roentgenography. Histologically, the tumor was composed of predominantly clear to lightly eosinophilic, polygonal cells with bland nuclei arranged in sheets and nests. Nuclear pleomorphism, necrosis, vascular invasion, and mitotic figures were not seen. The tumor cells were negative for oil-red-O and periodic acid-Schiff stains with and without diastase pretreatment on frozen and formalin-fixed sections, respectively. During immunohistochemical evaluation, the tumor cells were focally positive for cytokeratin and diffusely positive for neuron-specific enolase and chromogranin. Electron microscopy performed on paraffin block-retrieved tissue showed the presence of electron-dense, neurosecretory-type granules and variably sized vacuolated areas within the cytoplasm, the nature of which remained unclear. Intracytoplasmic glycogen or lipid were not identified. To our knowledge, this is the first report of pulmonary clear cell carcinoid tumor.